How to dynamically set default value in WTForms RadioField?

Question

I'm building a website with the Python Flask framework in which I use WTForms. In one form I've got a RadioField defined as follows:

display = RadioField('display', default='ONE')

This does not have any choices defined, because I do that lateron (which works perfectly fine):

myForm = MyForm()
myForm.display.choices = [('ONE', 'one'), ('TWO', 'two')]  # This works fine

I now want to set the default value for the RadioField after I set the choices for it. So I tried removing the default value from the definition (I'm not sure if 'ONE' is always an available choice) and I create the default value after I create the choices like I do above:

myForm.display.default = 'ONE'

Unfortunately this has no effect at all. If I set it manually in the Field definition like I do before it works fine, but not if I set it dynamically after I created the choices.

Does anybody know how I can dynamically set the default value for a RadioField in WTForms? All tips are welcome!

Answer 1

You need to run myForm.process() after adding the choices and setting the default property:

myForm = MyForm()
myForm.display.choices = [('ONE', 'one'), ('TWO', 'two')]
myForm.display.default = 'ONE'
myForm.process() # process choices & default

This is because the default is propagated to the field value (and, in the case of RadioField, the checked property) in the process method, which is called in the constructor.